Power brake assist using differential pressure actuated brake boosters is standard in the motor vehicle industry. These brake boosters have a diaphragm separating a vacuum chamber always open to a source of vacuum, such as created in an engine intake passage, and a working chamber normally provided with vacuum but controlled by valve apparatus responsive to input brake pedal actuation to allow atmospheric air therein and thus provide brake force assist through the diaphragm. In some systems, it is also known to provide a vacuum pump, either in place of the engine vacuum source or as a backup therefor, to provide a normal supplied vacuum.
But such brake boosters, which increase brake assist by decreasing vacuum in the working chamber, reach a limiting assist condition, called run-out, when the working chamber reaches atmospheric pressure. Although such boosters are generally designed so as to rarely reach the run-out condition, engine vacuum is limited and large, heavily loaded vehicles especially may require larger brake assist levels. Potential assist levels may be increased somewhat by the use of multiple diaphragm and chamber designs as are known in the art; but great size and weight penalties may result for assist capacity that is rarely used. It is desirable to provide a brake booster assist supplementing apparatus that provides such occasional greater than normal assist levels without incurring the size and weight penalties of a very large booster.